A multi-layer substrate having a conductive pattern and a resin film and a method for manufacturing the same are disclosed in, for example, U.S. Pat. No. 6,667,443-B2. In the multi-layer substrate, multiple thermoplastic resin films are stacked and multiple conductive patterns are formed among the resin films. Specifically, each resin film is made of thermoplastic resin, and each conductive pattern is formed on one side of the resin film so that a one-side conductive pattern film is formed. Multiple one-side conductive pattern films are stacked, and then, the stacked films are pressed and heated from both surfaces of the stacked films. Thus, the one-side conductive pattern films are bonded together.
In order to pack the multi-layer substrate densely, it is preferred that a surface conductive pattern disposed on the surface of the multi-layer substrate has a small thickness and a small area. Here, the surface conductive pattern of the multi-layer substrate is exposed outside. And an inner conductive pattern is disposed inside of the multi-layer substrate. Thus, the conductive patterns of the multi-layer substrate are composed of the surface conductive pattern and the inner conductive patter. However, in the above substrate disclosed in U.S. Pat. No. 6,667,443-B2, in order to simplify the construction of the multi-layer substrate, all conductive patterns are formed by an etching method. Specifically, a metallic film having a predetermined thickness is etched so that the conductive pattern is formed. Accordingly, the thickness of the surface conductive pattern is substantially equal to the thickness of the inner conductive pattern. In a case where the thickness of the surface conductive pattern is thin, connection reliability of the multi-layer substrate may be reduced by solder erosion when a land of the surface conductive pattern is bonded to an electrode of an electric device through the solder.